Cylinder Issue

[Sea Rider]

Would like to know which could be the cause for a 2 stroke 70 CC cylinder with low hours from new to present a light oval shape (arrows) which let's small light to pass through wall when ring is piston aligned inside cylinder and block is placed against a even light.





What happens if a new standard piston/ring or just ring combination is used, will engine lose bit of compression, same as if keeping the original piston & ring ? Would you say that new piston & ring with use will mold somehow to existing oval cylinder ? Or boring the cylinder to next piston/ring size is mandatory as a last resource for engine to work spot on.

Happy Boating

 

[Silvertip]

The cylinder is out of round due to wear. Keep in mind that every time a cylinder fires, the piston is thrust against the side of the cylinder due to the angle of the connecting rod all the way down the stroke. The only time the rod, piston, and crankshaft journal are 100% in line is at the top and bottom of the stroke. No! the ring will not assume the shape of the cylinder during wear. If it did, the old one would not have lost compression. It also appears there is a fairly large scratch on the bore but only you can confirm that. That cylinder needs boring and an appropriate piston and ring installed.

 

[Scott Danforth]

round piston in oval bore is no compression.

need to make the bore round again (bore and oversize piston, or bore and sleeve with standard size piston)

 

[racerone]

The ring gap does not show that the motor has a lot of hours on it.----Did you put the ring in the piston in the same position it would be when installed on the piston ?-Did you put the ring in the cylinder and square it up ?---And last but not least why was the motor taken apart ???

 

[boobie]

Agree with the above. Why was the motor taken apart ??

 

[racerone]

If you are not an experienced mechanic , then motors will fool you.---------Please note that motor parts are manufactured when at room temperature.------The wear on them occurs at operating temperature.-----Parts change shape / size when they are at operating temperature.-----So the check you are doing really does not add up to much.----Note, it does not take a large gap to be able to light through it.

 

[Sea Rider]

Thanks all,

Some updates, on first pic ring cut was not at right working angle. Now the ring cut as shown on second pic is at its correct working position, now ring light gap is shown between 11.30 to 1.45 o clock position when cylinder is placed against a even back light. Seems the ring is worn at that angle. So if a new standard piston/ring could make that engine to work acceptable could go that way.





Don't have any issues boring the cylinder and installing a 0.5mm oversize piston/ring kit, the down issue is that's the larger size available. Nothing in between standard and 0.5 mm next size. The engine has low worked hours as it's used as a emergency kicker on a J-24 sailboat. Wear cold be due to poor fuel/oir rate mix, bad oil quality, engine overheat, lower tail water intakes sucked floating debris ?

Can you believe the engine was dismantled because an idiotic pseudo mechanic wanted to have a look inside due to not being able to start the engine. The cylinder is even, no scratch, probably first pic said the contrary. So is it still possible for ring to have worn just at the stated upper portion ?

Happy Boating

 

[robert graham]

Maybe since it was an emergency kicker on a sailboat the rings never had time to seat/match the cylinder?....still needs running and breaking in?....Are there 2 rings on the piston and do they both have this light gap?....

 

[racerone]

Book shows 1 ring.----That motor was painted many different colours.------Marketed by Mercury , Johnson , Evinrude , Yamaha . Tohatsu and possibly others.

 

[boobie]

Get a standard size ring, run a glaze buster down the cylinder, measure the ring end gap and if okay put it back together.

 

[boobie]

I probably missed it, but who was the original manufacture of this motor ??

 

[Sea Rider]

It's a Tohatsu 3.5 HP engine. Parts have arrived, will bore the cylinder 0.5 mm to match next super size next week, should spring back to life.

Happy Boating

 

[Sea Rider]

A machine shop for car use couldn't bore my deformed cylinder wall, crankcase is too small to fit their machinery, ended at a motorcycle machine shop. Have done an impeccable work boring the cylinder to fit an oversized piston and ring set. Have added new piston pin, ball & needle bearings, seals and gaskets.





Compression specs states 78 PSI, with 3 hard pulls rises to 110 and with 6 to 125 and stays there. 78 PSI seems to be a typo error in manual specs. Tomarrow is D day, will need to break in the engine again for some hours, let's hope everything works as expected. Fingers crossed.

 

[Sea Rider]

Engine started second pull, great compression with oversized piston. Idled for 15 minutes at neutral with 25:1 oil/fuel mixture while being flushed with adapter through water intake found at lower crankcase,



Then after onto flat calm sea cond varying throttle from idle to 1/2 throttle for the next 3 hours. Engine pees as a teenager due to having polished water pump liner to incredible smoothness.



Mr. Rib has a huge hull drag for that small engine to push passing1/2 to 3/4 throttle, you can feel the hull drag exponential increment while giving more juice. Definitely will need to complete its full break in period at fast displacement speed with a much lighter boat with reduced hull drag as not to demand too much out of that small portable OB.

​Happy Boating

 

[bonz_d]

Happy to hear all is well! Congrats.

 

[steelespike]

Great job! There is a youtube video of a 2 hp Evinrude on a little jon boat with a light operator it actually planes.
Your 3.5 might really scoot on the right hull.

 

[Sea Rider]

Thanks Bonz and Steelespike, had a great time rising from the dead that nice 3.5 HP. BTW there is a theme related to this post which has not been answered, probabby a tricky inquiry. The post is named Lugging Dilemma, can find it some post belown this one. See if you can answer it the best you can.

Happy Boating